People should be replying to your marketing emails
A company called SendGrid has a great idea that they may be able to monopolize to increase their marketshare in the Email Service Provider space. Namely, it's using a very open set of API's to enable email sends. This may not itself be unique, but this combined with the idea that people on your opt-in marketing email list should ideally reply to an initial email to confirm that they want to be on the list, does two great things:
1) It is a double opt-in method, assuring you're using a gold standard in email marketing privacy practices
2) Because people reply to your email, it adds them to their whitelist (especially in yahoo and gmail), which increases your overall deliverability
